Valve sometimes readjusts ranks. From what I am seeing ranks a bit more volatile over the last couple days so you should notice more change one way or another.
Ranks are also percent based if I remember right so its possible that Valve also just changed that number for ranks which changed your rank. Like Oracle 4 used to be top X percent (idk 31%) and now Oracle 4 is top 21% (random numbers, i have no idea) which dropped your rank visually but your skill rating is the same.
But yeah, not a bug afaik.